The present invention generally relates to a method for measuring filler dispersion in uncured rubber.
To prepare rubber compositions for end use applications, rubber compounds are commonly mixed with reinforcing fillers, such as carbon black; processing oils and waxes, processing aids, such as zinc oxide and stearic acid; and other known additives, such as pigments, plasticizers, antioxidants, vulcanizing agents and accelerators, and the like. Typically, the rubber, fillers, and other selected ingredients are first mixed together in a xe2x80x9cmasterbatchxe2x80x9d to produce a uniformly blended compound in preparation for a second or final pass. In the final pass, the accelerators and curing agents are added so that the rubber compound may be vulcanized.
Quality control for rubber compounds focuses on the final mix, in order to assure that all ingredients have been incorporated into the compound in the proper proportions so as to produce a final product that will meet the quality requirements of the end user. Perhaps the most common quality control test of a final mix is the curemeter. The curemeter is typically run on compounds that have had accelerators and curing agents added, and are, thus, ready to go to final processing (i.e., shaping and vulcanization). The curemeter serves to identify those rubber compositions that are out of specification before they go into final processing, and also to make adjustments to the compounding process to assure correct future batches. In short, these tests are designed to xe2x80x9cfingerprintxe2x80x9d the batch (i.e., determine if it exhibits the desired properties at this testing stage). If the specific batch being tested matches the fingerprint, it is assumed that it will make an acceptable final product.
Processability tests are also employed in the quality control of rubber compounds. These tests are designed to determine the ability of the material to go through the intended forming processes, and are not specifically geared to assure that the final, cured rubber products will meet end-use specifications. These tests indicate the ability of compounded rubbers to be extruded, injection molded, or otherwise formed into final shapes. The Mooney viscosity test, the Mooney stress relaxation test, and DMRT instrument test are examples of such processability tests.
Curemeter and processability tests, while being useful for assuring quality end products, do not specifically address the mixing process and, more specifically, do not address the dispersion of the reinforcing fillers within compounded rubbers. Good filler dispersion is necessary because poor filler dispersion can lead to poor product appearance, poor processing and manufacturing uniformity, reduced product life and performance, and may also lead to a waste of raw materials and excessive energy usage during processing. Various methods have been developed to quantify the level of filler dispersion within compounded rubber. Of particular relevance to the method of the present invention is the reflected light measurement (RLM) method for measuring filler dispersion.
The RLM method for quantifying the level of filler dispersion within a compounded rubber composition will be generally appreciated with reference to FIGS. 1 and 2. FIG. 1 shows the cutting operation of a cutting blade 100 on a compounded rubber sample 102 containing reinforcing fillers 104. As the cutting blade 100 is advanced through the sample 102, it comes into contact with the fillers 104, and the fillers 104 are forced to move out of the path the cutting blade 100. This movement is represented by arrows A. Movement of the fillers 104 leaves behind either depressions 106 or a bumps 108 on the cut surfaces 110, 112 of the sample 102. The surface roughness resulting from the depressions 106 and bumps 108 thus relate to the level of filler dispersion, and the RLM method can be employed to measure the level of filler dispersion within the compounded rubber composition that provided the rubber sample 102.
Referring now to FIG. 2, the way in which the RLM method operates to quantify the level of filler dispersion within compounded rubber is generally depicted. Therein, a light source 116 sends beams of light 118, 120 toward the cut surface 110 of a rubber sample 102 that has been cut generally as described with reference to FIG. 1. The light beams 118 hit the cut surface 110 on an angle such that, when the light beams 118 reflect from either an indentation 106 or bump 108, the light 118 is reflected into a sensor 122 and, when light beams 120 are reflected from a smooth surface on cut surface 110, the reflected light is not picked up by sensor 122.
Light reflected back to sensor 122 thus indicates the existence of surface roughness, more particularly, the existence of a dispersed filler 104 within the rubber compound that provided the rubber sample 102. The amount and positioning of reflected light beams 118 picked up by sensor 122 is then compared to a standard set of images indicating dispersion ratings on a scale of 1 to 10, where 10 indicates very good dispersion.
However, in order to ensure that the results obtained from the reflected light measurement method are accurate and reliable, the cut surface 110 of the sample 102 must have a minimal amount of cut marks or smear marks, and the depressions therein should exist as a result of displaced filler, not entrapped air.
The present invention thus provides a method for measuring filler dispersion within uncured rubber through the reflected light measurement method, wherein cut rubber samples are prepared having minimal cut or smear marks and minimal depressions resulting from entrapped air within the rubber sample.
At least one method exists in the prior art for addressing these smear marks and entrapped air concerns. In this method, samples of compounded rubber are first pressed to remove entrapped air therefrom and, thereafter, the samples are pulled to about a 10% strain. Upon reaching this stretching point, the pulling action becomes static, and the sample is cut through the pressed portion. The stretching of the sample is performed in an attempt to prevent the creation of smear marks during cutting; however, it has been found that smear marks still result on the cut surfaces of the sample, due to the fact that, once the stretching action is allowed to go static, the rubber sample begins to relax and the cut surfaces do not sufficiently pull away from the cutting blade moving through the sample.
Thus, a need exists in the art for an improved method for cutting compounded rubber for reflected light measurement of the dispersion of fillers therein.
The need also exists for a device capable of carrying out the method disclosed herein.
In general, the present invention provides a method for cutting compounded rubber for reflected light measurement of the dispersion of fillers therein. The method includes subjecting a sample of compounded rubber to a dynamic pulling force; and cutting the sample for the purpose of analyzing the dispersion of fillers therein through reflected light measurement methods. Prior to subjecting the sample to a dynamic pulling force, the method may further include pressing the sample at the portion thereof that is to be cut in order to free the sample from entrapped air.
Pressing the sample at a portion thereof that is to be cut serves to minimize or eliminate the presence of entrapped air within the sample. Upon cutting the pressed sample, the cut surface will exhibit a minimal number of depressions, if any, resulting from cutting through air pockets therein. This will lead to a more accurate reflective light measurement of the dispersion of fillers within the sample because the depressions therein that reflect light back to the sensor will be the result of filler dispersion, not entrapped air.
Subjecting the sample to a dynamic pulling force during cutting also helps to ensure a more accurate reflected light measurement of filler dispersion, because cutting the sample in this manner either minimizes or eliminates the presence of smears on the cut surface. Particularly, as the cutting blade moves through a sample subjected to a dynamic pulling force, the cut portions of the sample tend to move away from the cutting blade due to the pulling force and the natural elasticity of the sample. The absence of smears on the cut surface of a sample also increases the accuracy of the reflected light measurement of filler dispersion, because smears present depressions and asperities on the cut surface that do not result from the presence of dispersed filler. The absence of smears helps to ensure that all light reflected back to the sensor reflects off of bumps or depressions created by dispersed fillers.
Notably, this method is preferably practiced on a masterbatch sample of rubber that is compounded with fillers. This method can serve as a direct indicator of the dispersion rating of the masterbatch and as an indirect indicator of the dispersion rating and physical properties of vulcanizates prepared from the masterbatch. The dispersion rating within the masterbatch can therefore be used as a process control measurement to indicate any needed changes in the mixing process. If the dispersion rating of the masterbatch does not correlate with desired end properties of a vulcanizate, the processing of the masterbatch can be adjusted to achieve the desired dispersion. Notwithstanding the foregoing, the method taught herein can also be employed with vulcanizate samples.
The present invention also provides a device for preparing a cut sample of compounded rubber in accordance with the method taught herein. Generally, the present invention provides a device for cutting compounded rubber for reflected light measurement of the dispersion of fillers therein. The device includes first and second clamps spaced apart across a gap for clamping a sample of compounded rubber across the gap. At least one of the first and second clamps is capable of moving relative to the other of the first and second clamps so as to alter the distance between the clamps for stretching a sample of compounded rubber clamped across the gap. A cutting blade is disposed in the gap for advancing through a sample of compounded rubber during the relative movement of the first and second clamps such that a sample of compounded rubber may be cut while being subjected to a dynamic pulling force.